This article describes the electric motor centrifugal switch or PTCs or PRDs - all devices used to cut the starting capacitor out of the motor circuit after the motor has reached a sufficient speed to overcome initial starting resistance torque.

Define & Explain Electric Motor Centrifugal Switch, PTC or PRD devices

Electric motor capacitors are devices that store or accumulate an electrical charge that can be released at high voltage to get an electric motor running at start-up (starting capacitors) or that help keep a motor running once it has started (smaller, run-capacitors).

At CAPACITOR SIZE DETERMINATION for ELECTRIC MOTORS, we explain that once a starting capacitor has provided the necessary boost to get the electric motor spinning, in order to avoid electric motor damage or damage to the capacitor itself, the starting capacitor has be disconnected from from the electrical circuit, leaving the motor to run on normal operating current + the run capacitor.

Typically one of three different devices, a centrifugal switch or possibly a PTC (Positive Temperature Coefficient device) or PRD (Potential Relay Device) does this by dropping the start capacitor from the motor's electrical circuit once the motor has spun up to operating speed.

Electric motor start switch / centrifugal switch

A centrifugal switch connects the A/C electrical power to the motor to the start winding on the stator (and/or to the start capacitor) until the motor has reached a speed typically of 75-80% of its full run speed .

Centrifugal switches for electric motors like the switch shown above are sold with two to 6 poles and are specified as having specific-range of switch CUT-OUT (open or off) and CUT-IN (close or on) speeds.

For small electric motors like those used in most HVAC applications as well as in other small equipment that runs on 120VAC or 240VAC, the electric motor full speed is usually 1725 or 3450 rpm, though some equipment may use variable speed motors as well.

• High speed electric motor centrifugal switch open/close speeds: A centrifugal switch like the ones shown at page top and at above-left will open ("throwout") or ("turn off the capacitor") at about 2800 rpm for a 3450 rpm electric motor.
  
  In more detail: a centrifugal switch for a modern "high speed" electric motor rated at a running speed of 3450 rpm under full load might cut in or close (turn on the capacitor) from 1000-2000 rpm, and might open at speeds between 2600-3000 rpm.
• Low-speed electric motor centrifugal switch open/close speeds: The centrifugal switch will open (turn off the capacitor) at about 1400 rpm for a 1725 rpm electric motor and will be closed at speeds from 0 to about 1200 rpm [est].

What is the Function of the Centrifugal Switch in an Electric Motor?

When the motor gets up to speed and the centrifugal switch opens to take the starting capacitor out of the circuit. We only need the start capacitor to help a motor start spinning from its off or still position.

As is often the case, there is a second separate (smaller) run-capacitor, that remains in the circuit to help keep the motor spinning more-smoothly and at greater efficiency.

Induction Motor Starting-Boost Design Options

In our more-detailed induction-type electric motor wiring diagram shown just below, the centrifugal switch, a model that is normally closed, opens at speed to cut both the start capacitor and the start winding from the motor's circuit.

Watch out: If your electric motor uses this design an dif the centrifugal switch fails to operate the starting capactor will burn up or even explode and/or the start winding may overheat and be damaged.

Below our elecric motor drawing shows a different capacitor start/run design in which only the startign capacitor is cut from the circuit when the motor approaches run-speed. In this design the start winding remains energized as the motor continues to run. At that point the start winding, or secondary winding, is functioning as an auxiliary winding helping smooth the motor's rotation.

In this design if the centrifugal switch fails to open when it should (at higher motor speed) the starting capacitor will be destroyed.

What Happens if the Centrifugal Switch in an Electric Motor Fails?

Watch out: If the centrifugal switch is broken in the "switch-open" position, it will never connect the start capacitor to the motor's start winding, and the motor won't start and the main or "run" winding will overheat but may not fail.

Watch out: If the centrifugal switch is broken in the "switch-closed" position, it will keep the starting capacitor in the circuit and connected to the start winding continuously even after the motor has started. The result is likely to be burnt-up motor start-windings. The centrifugal switch on a typical single-phase or split phase electric motor is designed to OPEN at 70-80% of the motor's full speed.

Watch out: A "catastrophic capacitor failure" of a motor starting capacitor in which the start capacitor "explodes" or bursts can occur when the start capacitor remained engaged too long - perhaps because a centrifugal switch inside the motor did not disengage the starting cap circuit when it should-have.

Is the Problem with the Motor Starting Capacitor or with the Centrifugal Switch in the Motor?

Without actually testing the motor start and run capacitor (or simply trying a new one) we can't be sure without more-extensive testing if the "motor won't run" problem is

• A failed starting capacitor
• A failed, damaged, shorted, burnt motor winding
• A failed centrifugal switch

We provide ELECTRIC MOTOR DIAGNOSTIC GUIDE - to help sort out these different causes of electric motor failure

Watch out: If the starting capacitor has failed the symptom is that the motor won't start. You may hear it humming or observe that it's getting hot. Turn the motor off right away If you observe this, in order to avoid damage while waiting for repairs.

Where to Find the Centrifugal Switch in an Electric Motor

Check out the rear bell housing of the motor to see if the motor uses a centrifugal switch to switch the start / run capacitor or other windings in and out of the circuit at a specific RPM.

If a centrifugal switch is present, check that its switch contacts are not welded closed or contaminated with dirt and grease. The switch mechanism should can move freely. - WikiHow [36]

Not all motors use a start capacitor and centrifugal switch

Starting capacitors or electric motor starting capacitors (or motor start boosters) are often present on large single phase air conditioning compressors, as found on home air conditioning units, or on occasion on blower motors or even fan motors.

Electric motor starting capacitors are only very rarely present on small refrigeration compressors, such as those in refrigerators, and as far as we know, never present on 3-phase power systems.

We give more detail about failed electric motor starting capacitors, and we explain possible visual diagnosis of a failed starter capacitor (bulged ends) without having to perform electrical testing,

at HARD STARTING COMPRESSOR MOTORS.

 

PTC-devices Can Drop the Start Capacitor from a Circuit

When a capacitor-start, induction-run electric motor such as is used on an air conditioner or heat pump must be hermetically sealed, we can't use a centrifugal switch. Instead we use one of the other devices described here: PTC devices, PRD relays, solid state starting relay, hot wire relays, or current relays.

Positive Temperature Coefficient devices are the traditional means of dropping the capacitor from the motor circuit once the motor has successfully started.

PTC devices are basically a thermistor-type device, using a tiny electric heater that use the change in electrical resistance of the heating element to open an electrical switch that removes the start winding in the motor from the run circuit. The switch opens in less than a second after the motor has started.

PTC devices have the advantage of being simple and avoiding the need for more complex electrical wiring of a motor starting system.

Supco explains that this device is unable to sense whether or not the motor has successfully started, and if the motor does not start, several minutes are needed to let the heater cool down before the motor restart can be attempted again.

This cool-down time provides a safety margin that helps protect against burning out the windings of a hard-starting motor.

PRD Potential Relay Device + Hard Start Capacitors Boost Starting Torque for HVAC Motors on TXV-based systems

Potential Relay Devices use voltage sensing (The Supco method) or current sensing devices (two different approaches) to determine when to release the starting capacitor from the motor run circuit.

Supco points out that "The electronic potential relay is inherently more reliable and precise than the older type mechanical potential relay." [1]

Both of these start capacitor control approaches work fine, and typical HVAC or residential appliance motor repairs the technician won't need to consider which method is being used to control the capacitor as long as she/he follows the manufacturer's recommendations on the product for its selection and use.

SUPCO offers this explanation of why a potential relay hard start kit would be used:

Most single phase air conditioners and heat pumps use non-bleed thermostatic expansion valves (TXV’s or TEV's) to control refrigerant.

A problem with TXV’s occurs when a reciprocating compressor shuts off, refrigerant pressures don’t fully equalize. Pressures do equalize in scroll compressors; which, typically do not need hard start kits for this condition.

In a reciprocating compressor, the discharge pressure will drop to about 150 psig and the suction pressure will rise only to about 100 psig.

When the compressor tries to start, there’s too much load for the starting motor torque to overcome. This is especially true if the supply voltage is low. To increase starting torque, a start assist device can be used.

When a TXV is used, a potential relay hard start kit is employed. This will increase starting torque by a minimum of 300 % over using just a run capacitor.  

... Systems with capillary tubes or fixed restrictors usually don’t need a full hard start kit, unless the compressor bearings are tight. In such cases, the compressor is probably near the end of its useful life anyway. - SUPCO Hard Starts booklet cited below.

The SUPCO booklet we cite below provides wiring connection details for 3-wire and 2-wire connected potential relay / hard start kits.

 

Hot Wire Starting Relay Devices

The following description of hot wire start relays and current relays is general and not technically exhaustive.

We welcome further comments or suggestions regarding these electric motor controls.

An alternative control, the hot wire relay, operates as both a start relay to releasae the start capacitor and/or start winding and also as an electrical overload or protection device. The hot wire relay works by making use of a resistive wire that causes a spring-loaded contact to open. During motor start-up high current flows through this relay and its resistive wire, causing the wire to expand. When sufficiently expanded, the wire opens a spring-loaded start-winding contact switch, disconnecting the winding from the circuit.

Current Relay Devices

A current relay is similar to the hot wire relay but rather than relying on temperature and an expanding wire to open the contacts of a switch, the current relay makes use of a magnetic field to open or close the device. This is a NO or Normally-Open switch whose coil connected in series with the motor run-winding and whose contacts connect in series with the start winding. With the switch open the start-winding is disconnected, when current is applied high current flows through the run-winding, producing a magnetic field in the coil that pulls the contacts closed to connect and operate the start winding.

Once the motor has begun spinning the run-winding current is reduced, the switch then opens and the start winding is disconnected. But as the motor is now running the current flow is reduced so the switch remains in its NO position.

 

How to Diagnose & Repair the Electric Motor's Centrifugal Switch

If your electric motor uses a centrifugal switch to cut out the "start" capacitor once the motor is up to speed, a failure of that switch can cause motor failure.

Make a visual inspection of the switch, normally at the rear or "bell housing" end of the motor.

Common Electric Motor Centrifugal Switch Defects

• Centrifugal switch contacts are dirt clogged - even a small bit of dust, oily-debris, or crud can cause trouble
• Centrifugal switch contacts don't move freely
• Arc burns on the centrifugal switch contacts
• Centrifugal switch contacts are welded together
• Motor runs backwards: if the centrifugal switch is sticking you can get random choice of run direction when the motor starts-up again.
  
  Some electric motors use a potential relay instead of a centrifugal switch. If the pot. relay is bad the effect of random start direction could still occur.
• The problem isn't the centrifugal switch but instead is a bad start or run capacitor
• The problem isn't the centrifugal switch but instead is a motor winding that shorts or opens as the motor's rotating speed increases

Motor Centrifugal Switch Diagnostic Tips

Thanks to a reader, Paul J. Ste. Marie we can add to the centrifugal switch diagnostic checkpoints above these comments and diagnostic tips:

A centrifugal switch as used in some electric motors uses the centrifugal or rotating force of the motor to open or close an electrical switch.

Electrical current applied to a start winding causes an electrical field placed at an angle to the field produced by the main or "run" winding to provide an initial rotating force to start the motor.

In these split-phase electric motors a centrifugal switch is used to switch off electrical power to the start winding once the motor has gotten up to a minimum speed of about 75% of its full run velocity. In essence the centrifugal switch turns off the start circuit and leaves the motor operating from its run-circuit alone.

This application of centrifugal switches is common in split-phase or "induction-start / induction-run alternating current (AC) electric motors that do not require a high starting torque, such as small power tools, grinders, etc. - citation. - Heinecke, Kevin (2015)

atReferences or Citations .

Ste. Marie pointed out that

Another common reason for a CS or CSCR motor to fail to start is a bad centrifugal switch. If the switch contacts are dirty, the motor can fail to start.

If the switch freezes closed, the start capacitor will overheat. - P.S.M. 10 Jan 2015

Watch out: some damaged motor windings will test out just fine on the bench with the motor disconnected completely, but when the motor begins to spin a damaged wire in the winding may, due to the centrifugal force of rotation, open, causing the motor to run poorly or to stop completely or to chatter.

More detailed VOM tests on motor leads and windings are given in our diagnostic table found

at ELECTRIC MOTOR DIAGNOSTIC GUIDE

Definition of Primary vs Secondary Electric Motor Windings

Most single phase motors have two sets of windings.

The main or primary windings are directly connected to the power lines while the motor in running. The second windings are usually thinner wires physically offset from the main windings inside the motor.

The purpose of the secondary windings is to provide directional information and an initial strong kick to get the motor started turning. Once the motor is started, the main or primary windings can keep the motor running just fine.

Less common, these secondary windings are directly powered from the power lines through a run capacitor that provides a continuous time or phase shift to the windings.

Far more common, the secondary windings and capacitor are powered through a centrifugal switch that is closed for approximately 1/2 second on starting.

As the motor gets up to 2/3 speed, the centrifugal switch opens and disconnects the secondary windings.

This switch is usually behind and part of the connection plate where you attach the power cables in the end of the motor.

Any little piece of dust can keep this switch from closing when the motor is stopped.

At this point, the motor just sits there and hums

not knowing which way to go or how to get started.

Simply knocking this one little dust particle off might make the repair, and the motor will run fine henceforth. Most of the time you have to pull the back end of the motor off to get to this switch. - Courtesy of an InspectApedia.com reader

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: How does the start capacitor get turned off when the motor has reached its rated speed?

2021/07/07 How start asked:

How different start capacitor off state after motor in rated speed

Moderator reply:

On many motors a centrifugal switch built into the motor and held closed by springs keeps the start capacitor in the motor power circuit until the motor has reached its rated speed. At that point the rotating speed of the motor is sufficient to open the centrifugal switch against its spring pressure, thus taking the starting capacitor out of the circuit, leaving the run capacitor in use.

To give you an idea what the centrifugal switch looks-like, below we show a Speedaire (brand) Centrifugal Switch Set Mfr #: PN22N031G as listed for sale at Zoro.com, Zoro #: G8641053

This Q&A were posted originally at CAPACITOR SIZE DETERMINATION for ELECTRIC MOTORS

On 2019-07-24 by (mod) - start capacitor drops out at 50% to 80% of BEMF

Thank you KD for this helpful post. If you are a professional in the field who wants to be acknowledged with a link and identification let us know. We are happy to cite and refer to technical contributors.

On 2019-07-24 y KD Grayson

A Run capacitor 's terminals are wired to the Run and Start motor leads, the start capacitor also goes to the Run and Start leads but only briefly, the Start capacitor must remain in the system for a minimum of 50% of the BEMF (back electro motive force) and not beyond 80% of the BEMF.

Should the Start capacitor drop out below 50% of BEMF the motor may stall, if the Start capacitor stays in beyond 80% of the BEMF you risk exploding the Start capacitor.

On 2018-07-21 by (mod) - success with capacitor testing - loose wire found at centrifugal switch

Fantastic!

I'll keep your comment and add this tip to the article - I'd forgotten about the obvious: check for a loose connection. That can look just like a bad capacitor.

On 2018-07-21 by Jesse

(Correcting some awful voice to text translation)
Thanks for this article. I didn't have a capacitor tester at the time (I do now) but

because of this article I went ahead and checked the Starter capacitor centrifugal switch that's keyed onto the main stator shaft and guess what...it was loose.

After all that time chasing down the capacitor rabbit hole, in the end it was a loose Phillips head key screw. (For an air compressor). Thanks!

Posted originally at CAUSES of HARD STARTING ELECTRIC MOTORS [live link given at Recommended Articles - below]

On 2018-06-12 - by (mod) -

Sham

I'm only guessing, but I think that the problem is probably in a switch or switch wiring since the operation of the motor is not causing the circuit breaker or GFI to trip.

This Q&A were posted originally at ELECTRIC MOTOR DIAGNOSTIC GUIDE

On 2018-06-12 by Shammock112

I have a 3/4 HP single phase AC motor connected through a ground fault breaker used for a boat lift.

The motor runs fine in the "Up" direction but trips the breaker about 3-4 seconds in when switching to reverse. Everything was working fine until a bird decided to build a nest on the centrifugal switch end and I found dirt-dobbers [mud dauber wasps - Ed] making homes in the windings.

The motor wouldn't do anything but hum when I turned the switch either up or down. I took the motor apart and cleaned it up but I have the breaker tripping problem. Do you have any suggestions?

On 2015-04-06 by (mod) - motor spins but buzzes - is this a bad centrifugal switch?

What an interesting question, Peter.

If you have an ammeter you can check to see if the motor continues to draw current after it's switched off - if so I would suspect a bad motor control switch.

But there are centifugal switches that open in some motors that switch the motor from a "start" to a "run" circuit once the motor has gotten up to speed. A sticking switch could explain what you are hearing.

On 2015-04-06 by Peter

Hi
I have a new large bandsaw in my furniture workshop, working on single phase, 230 volts AC (in France). The machine starts and works fine but after switching off, there is a buzzing sound (not humming) from the stopped motor, lasting for about 15 seconds. The capacitor is ok when checked with a multimeter.

Are there electrical points within the motor housing which work centrifugally to start the motor spinning, and this noise is indicating that the points are not disengaging fully when the power is shut off?

On 2015-01-10 by (mod) -

Paul

Thank you very much - you're quite right, of course. I have added bad centrifugal switches to our list of motor defects both in the article above - it already appeared in our separate

article ELECTRIC MOTOR DIAGNOSTIC GUIDE but it was not explained on this page.

Thanks to you that point appears here. If you want to be further identified in the article let me know by email, or if you prefer to remain anonymous let me know that and I'll remove your name.


Working together makes us smarter.

On 2015-01-10 by Paul J. Ste. Marie

Another common reason for a CS or CSCR motor to fail to start is a bad centrifugal switch. If the switch contacts are dirty, the motor can fail to start. If the switch freezes closed, the start capacitor will overheat.

Posted originally at CAUSES of HARD STARTING ELECTRIC MOTORS [live link given at Recommended Articles - below]

...

Continue reading at ELECTRIC MOTOR DIAGNOSTIC GUIDE - home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• BASIC ELECTRICAL TESTS for BURNED OUT COMPRESSOR MOTORS
• CAPACITOR TYPES, for MOTORS
  • CAPACITORS for HARD STARTING MOTORS
  • CAUSES of HARD STARTING ELECTRIC MOTORS
  • CAPACITOR SIZE DETERMINATION for ELECTRIC MOTORS
  • COPELAND ELECTRICAL HANDBOOK [PDF]
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• ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD
• ELECTRIC MOTOR DIAGNOSTIC GUIDE - home
• ELECTRIC MOTOR HORSEPOWER & CIRCUIT WIRE SIZE
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• ELECTRIC MOTOR RUN DIRECTION
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• HARD STARTING ELECTRIC MOTOR CAUSES
• HUMMING MOTOR SOUNDS

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